Sometimes while testing file systems or applications you want to simulate some errors on the disk level. The first time I heard about this need was from Baptiste Daroussin during his presentation at AsiaBSDCon 2016. He mentioned how they had built a test lab with it. The same need was recently discussed during the PGCon 2019, to test a PostgreSQL instance. If you are FreeBSD user, I have great news for you: there is a GEOM provider which allows you to simulate a failing device.